At present, foods are preserved commercially by thermal processing including ultra high temperature (UHT) or high temperature short time (HTST) processes. Extending shelf life of foods by heat treatment is not only energy intensive but in most cases, adversely affects the flavour (burned, cooked and scorched), chemical composition (organoleptic properties) and nutritional quality (vitamins) of treated foods. High-voltage pulsed electrical field treatment is currently being explored as an alternative and is the most promising technology under development as a non-thermal process to substitute for food additives or energy intensive thermal preservation methods. This also minimizes the loss of natural vitamins and flavours. The big advantage is the energy saving of the process.
The feasibility of using high voltage electric pulses for pasteurization of food has been investigated by researchers in Canada, USA, and Japan. Experimental studies have shown that pulsed electric fields can induce moderate to significant microbial inactivation in various aqueous solutions (Sato et al., 1994; Bushnell et al., 1991, 1993, 1995a,b; Zhang et al., 1994a, 1994b, 1994c, 1995b; Qin et al., 1995; Ho et al., 1995; Marquez et al., 1997, Jayaram et al., 1992; Gupta and Murray, 1989). The application of high voltage electric pulses to pasteurize liquid food products is thought to be a non-thermal and energy efficient process as compared to traditional thermal pasteurization. Moreover, researchers have found evidence that the same technology can be applied to other areas such as enzyme inactivation, product yield improvements, and semisolid/particulate/solid foods pasteurization (Vega-Mercado et al., 1995; Ho et al., 1997).
Pulsed power refers to the general technology of accumulating energy on a relatively long time scale (pulse charging, slow systems) and then compressing that energy in time and space to deliver large power pulses (pulse discharging, high speed systems) to a desired load. These pulses may last anywhere from hundreds of picoseconds to tens of seconds. Most of the studies in the past have been conducted using small-scale, batch mode treatment systems. Furthermore, only a few studies have treated real food products on a pilot-scale, continuous flow operation. The process parameters used for batch processing have had a very wide range: d.c. voltage: 2.5 to 43 kV; electric field strength: 0.6 to 100 kV/cm; electrode distance: 3 to 77 mm; pulse width: 1 .mu.s to 10 ms; pulse frequency: 0.2 to 50 Hz; number of applied pulses: 1 to 120; and process volume: 0.5 mL to 1.6 L. Properties of the suspending media (electrical conductivity, pH, and compositions), process temperature, and microbiological conditions were sometimes not reported by the researchers. Based on all the available studies, the microbial reduction rate was found to range from a moderate 1-3 log cycles to a significant 6-9 log cycles, and seemed to be a function of various process parameters, conditions, and procedures.
Dunn and Pearlman (1987, 1989) patented a horizontal continuous flow system. The treatment chamber had multiple electrode zones which were electrically isolated from each other by insulation. The electrodes were separated from the fluid food by an ion membrane and electrolyte. A continuous potential was applied to each electrode. Thus, the fluid that flowed through the chamber would experience pulsed treatments at specific zones. They depicted the need to use pre-heat treatment as an aid. Experimental results were not reported. Although the concept is unique and the treatment time (flow rate) can be easily controlled, the chamber is somewhat bulky to operate and the energy savings are limited as high voltage is constantly supplied to the unit, and high energy pulses were used.
Bushnell et al. (1993) also patented a horizontal continuous flow system similar to that of as Sato et al. (1994). The high voltage inner electrode was held in position by a metallic connecting rod (to the pulser) which was surrounded by insulation. The inner electrode had tapered surfaces at both ends to minimize eddying or flow stagnation. The sample entered from the top (side hole of outer electrode) and came out from the middle of the pipe. They identified the need to use pre-heat treatment as an aid. Experimental results were not reported. Again, the chamber is bulky in configuration, and the existence of low electric field regions may hinder proper microbial control.
Qin et al.,1995 and Zhang et al., 1995a disclosed two chambers for continuous processing. The first one is a parallel plate chamber based on their batch chamber. The sample flows through the horizontal test chamber in a series of U-shaped channels. The electrode gap is 0.51 or 0.95 cm giving a volume of 8 or 20 mL. The operational parameters were 35 to 70 kV/cm, 2 to 15 .mu.s pulse width, 1 Hz pulse frequency, and a flow rate of 0.6 to 1.2 L/min. Circulation of water around the circular stainless steel electrodes through jackets was implemented for cooling purposes due to the use of high energy pulses. The electric field strength and flow profile may be difficult to monitor with the use of U-shaped channels. The second one was a coaxial treatment chamber. They analyzed the chamber designed by Bushnell et al. (1993) and determined several local field enhancement points (Zhang et al. 1996). They then used an electric field optimization technique based on finite element method to modify the assembly by changing the electrode gap along the chamber. The operation parameters were 50 to 80 kV/cm, 2 to 6 mm electrode distance along the chamber, 2 to 15 .mu.s pulse width, 1 Hz pulse frequency, and a flow rate of 2 to 10 L/min. Cooling jackets were attached to both electrodes to remove additional energy supplied by high energy pulses.
For the coaxial chamber continuous system, Qin et al. (1995) reported a reduction of 7 log cycles in simulated milk ultrafiltrate inoculated with an initial count of 8.times.10.sup.8 cfu/mL E. coli. A 6-7 log cycle reduction was also obtained with commercial apple juice inoculated with a Saccharomyces. The process parameters used were exponential decay pulses, 50 kV/cm electric field strength, :2.5 .mu.s pulse width, 1 Hz pulse frequency, 0.6 cm electrode gap (30 mL volume), and 2 pulses applied to the food. Treatment temperature was controlled between 22 to 34.degree. C. Flow rates were not reported. Vega-Marcado et al. (1996) inoculated pea soup (semi-liquid) with E. coli and B. subtilis separately at 1.times.10.sup.7 cfu/mL, and obtained a maximum reduction of 6.9 log cycles for E. coli and 5.25 log cycles for B. subtilis using 33 kV/cm electric field strength, 0.5 L/min flow rate at 4.3 Hz pulse frequency (30 pulses), and 55.degree. C. treatment temperature. Zhang et al. (1994a, 1994b) inoculated a pure culture of S. cerevisiae in commercial apple juice (not cider), and obtained a microbial reduction from an initial 10.sup.7 cfu/mL to a final 10.sup.3 cfu/mL. The conditions were 12 kV/cm for 20 pulses. The energy requirement was 260 J/pulse or 208 J/mL for a 25 mL batch. An auxiliary cooling system was also required to maintain a 4.degree. C. process temperature. Physical and chemical property measurements were not reported.
Yin et al. 1997 have also reported studies on continuous treatments using a so-called "co-field flow" model treatment chamber, where the flow direction of the liquid medium was parallel to electric field. However, details on the treatment systems and process conditions were not clearly specified.
Previous inventors used high energy pulses and equipment to generate these. There are various problems with high energy pulses. These pulse generators are generally very costly due to the need to generate and control high energy pulses. In some systems, generally there is no energy saving, and sometimes higher energy is needed to treat foods compared to conventional thermal methods. Since large energy is applied to food, extra energy is required to be removed in order to control food temperature. In some cases, process temperature should be at least 45.degree. C. for successful pasteurization. Such devices are bulky due to insulation requirements, as well as special electrical installation is needed due to larger power requirements. There are some problems with fouling of electrodes when foods containing proteins are treated. Electrode erosion, electrolysis of food, and breakdown of food generally occur due to high energy content of pulses.
When using high energy pulses, it is difficult to treat foods with high electrical conductivity as high current generated will create electrical breakdown of food. There is therefore a great need for a non-thermal and low energy per pulse electrical method for inactivating microorganisms that can be broadly applied to foodstuffs with varying conductivities that is economical, compact, energy efficient, safe, environmentally acceptable, and which does not significantly affect nutrition, texture and flavour of the treated food.